The present invention relates to optical branching and coupling devices. More particularly, it relates to an optical branching and coupling device of the type provided to a terminal of a subscriber's system for branching laser light having two different wavelengths, which is input from an optical fiber, into two rays of light of individual wavelengths so as to receive optical signals of each ray, and coupling light from a light-emitting element such as a laser diode to an optical fiber so as to transmit optical signals thereof.
There has hitherto been known an optical branching and coupling device (hereinafter referred to as "coupler" simply) 51 of the type shown in FIG. 2. To this coupler are connected a first optical fiber 52 for an input of light having two different wavelengths (for example, 1.31 .mu.m and 1.55 .mu.m) and for an output of light of 1.31 .mu.m-wavelength (hereinafter referred to as "1.31 .mu.m-ray"), and a second optical fiber 53 for an output of light of 1.55 .mu.m-wavelength (hereinafter referred to as "1.55 .mu.m-ray") and for providing a connection with a terminal unit such as a television set. Further, in the coupler 51 are provided a half mirror 54 on the side of the first optical fiber 52 and a high pass filter 55 on the side of the second optical fiber 53.
The half mirror 54 is positioned at an angle of 45.degree. with respect to an optical axis (hereinafter referred to as "first optical axis") 57 which runs from the first optical fiber 52 and passes through the center of a first collimating lens 56. The half mirror 54 has a function of permitting about a half of the light quantity of each of the aforementioned rays to pass therethrough and of reflecting the rest.
On the other side, the high pass filter 55 is positioned at an angle of 45.degree. with respect to an optical axis (hereinafter referred to as "third optical axis") 59 which passes through the center of a first converging lens 58 and reaches the second optical fiber. The high pass filter 55 has a function of permitting 1.31 .mu.m-ray to substantially pass therethrough and of substantially reflecting 1.55 .mu.m-ray. The two components 54 and 55 are disposed so that the respective planes thereof would be normal to each other. Therefore, an optical axis (hereinafter referred to as "second optical axis") 60 which runs from the half mirror 54 to the high pass filter 55 crosses each of the optical axes 57 and 59 at right angles.
On the side of the back of the half mirror 54 (on the side opposite to the first optical fiber 52) and substantially on the first optical axis 57 there is provided a light-emitting element 62 for emitting 1.31 .mu.m-ray. In general the light-emitting element 62 is composed of a laser diode. A second collimating lens 61 is interposed between the half mirror 54 and the light-emitting element 62.
On the side of the back of the high pass filter 55 (on the side opposite to the half mirror 54) and substantially on the second optical axis 60 there is provided a light-receiving element 64 composed of a photodiode or the like. A second converging lens 63 is interposed between the high pass filter 55 and the light-receiving element 64.
When 1.31 .mu.m-ray and 1.55 .mu.m-ray are to be input from the first optical fiber 52 to the coupler 51 thus arranged, and when an optical signal is to be transmitted from the light-emitting element 62 to the first optical fiber 52, the following three optical paths appear.
Path (1): receiving 1.31 .mu.m-ray
The 1.31 .mu.m-ray follows the path: first optical fiber 52.fwdarw.half mirror 54 (reflected).fwdarw.high pass filter 55 (transmitted).fwdarw.light-receiving element 64. Path (1) allows receiving personal information such as telephone information and telecopier information from a telephone station, digital-to-analog converting it, and receiving the converted signal. 1.55 .mu.m-ray on path (1) is attenuated by 30 dB as will be described later and, hence, the intensity thereof becomes sufficiently small as compared with that of 1.31 .mu.m-ray (refer to Table 1).
Path (2): receiving 1.55 .mu.m-ray
1.55 .mu.m-ray follows the path: first optical fiber 52.fwdarw.half mirror 54 (reflected).fwdarw.high pass filter 55 (reflected).fwdarw.second optical fiber 53. Path (2) allows receiving information from mass media such as television programs and is adapted to connect to a terminal unit. A small amount of 1.31 .mu.m-ray is included on path (2) and imposes a burden on a decoder for reproducing signals.
Path (3): transmission of 1.31 .mu.m-ray
1.31 .mu.m-ray follows the path: light-emitting element 62.fwdarw.half mirror 54 (transmitted).fwdarw.first optical fiber 52. Path (3) allows transmission of personal information such as telephone information and telecopier information.
For the three paths, the loss of each ray and the branching characteristic throughout the coupler 51 in accordance as the advance of each ray are shown in Tables 1 to 3, where T represents a transmittance, and R a reflectance.
TABLE 1 ______________________________________ Path (1): receiving 1.31 .mu.m-ray Wavelength High pass Representation (.mu.m) Half mirror filter Total in dB ______________________________________ 1.55 R 0.508 T 0.002 0.001 -30.0 1.31 R 0.483 T 0.98 0.473 -3.2 ______________________________________
TABLE 2 ______________________________________ Path (2): receiving 1.55 .mu.m-ray Wavelength High pass Representation (.mu.m) Half mirror filter Total in dB ______________________________________ 1.55 R 0.508 T 0.998 0.507 -3.0 1.31 R 0.483 T 0.02 0.009 -20.2 ______________________________________
TABLE 3 ______________________________________ Path (3): transmission of 1.31 .mu.m-ray Wavelength Representation (.mu.m) Half mirror in dB ______________________________________ 1.31 T 0.52 -2.8 ______________________________________
With the coupler 51, as apparent from Tables 1 to 3, 1.31 .mu.m-ray and 1.55 .mu.m-ray, input from the first optical fiber 52, are each half-reflected by the half mirror 54 and fed to the high pass filter 55, and the remaining half is transmitted through the half mirror 55 and lost. As a result, problems arise such that the power of 1.55 .mu.m-ray to be output is reduced to about a half of the input power thereof, and that 1.31 .mu.m-ray included is removed with a low precision (i.e., poor branching characteristic).
The present invention has been attained so as to overcome the foregoing problems. It is, therefore, an object of the present invention to provide a coupler wherein the loss of a second ray (for example, 1.55 .mu.m-ray) coupled from an input optical fiber (for example, the first optical fiber 52 in the aforementioned prior art) to an output optical fiber (for example, the second optical fiber 53 in the aforementioned prior art) is substantially reduced, while at the same time the wavelength-branching characteristic for branching the second ray and a first ray (for example, 1.31 .mu.m-ray) from each other when to be output to the output optical fiber is remarkably improved.